Epitope mapping in TSPAN7 a novel T1D related beta cell autoantigen

BACKGROUND: Tetraspanin 7 is an integral membrane protein that is expressed in the pancreatic beta cells. It is also an autoantigen target in type 1 diabetes. Autoantibodies to tetraspanin7 are found in 35% of patients at disease onset as compared to < 1% of healthy control subjects. Since tetraspanin 7 span both cytosolic and extracellular compartments, we asked where the major autoantibody epitopes of this beta cell protein were located. METHODS: We prepared constructs that contained separate domains of the protein, expressed these as Nluc tagged proteins and measured patient autoantibody binding to each using a Luciferase Immunoprecipitation Assay. Epitopes in tetraspanin7 positive sera were mapped down to two soluble domains of the protein, extracellular domain 2 and cytoplasmic domain 3 (C3). We then performed alanine scanning of the C3 cytoplasmic domain and revealed the importance of amino acids 239 and 243-247 for binding antibodies from patient sera. RESULTS: We define an autoantibody epitope within a cytoplasmic loop of tetraspanin 7, thus revealing a novel T1D relevant epitope.